The idea I was rolling with is that it's less about the ethics of enslaving others and instead on seeing slavery as a corrupting evil on political institutions(at first). From the perspective of most of the Atlantic Seaboard, the Caribbean sugar planters have monopolized what political influence the colonies have in London and are encouraging Parliament to act to their exclusive benefit with respect to trade, more say in politics, etc. that polarizes New World politics between the Caribbean planters(and those who seek to export this society onto the rest of the New World) versus those who resent the disproportionate power, wealth, and influence of these planters. These resentments manifest themselves during the Enlightenment era in OTL's USA via a stronger critique of slavery. Critiquing slavery becomes the go-to vehicle for the intelligentsia to take shots at the Caribbean planters and their perceived excess; as a result, slavery is far more universally decried on many fronts across the entire American political spectrum of the early Revolution that alienates much of the South bar Georgia, which is a free colony.
As a result of these butterflies and greater hostility to slavery, the planters are far more pro-British. Once the ARW comes around, this polarization leads to a different perspective from the American Revolutionaries who are far less shy on decrying slavery because of the Carolinas being solidly behind the British as well as large parts of Virginia. Slavery and the planters are demonized as in cahoots with King George and slavery becomes synonymous with moral depravity, not because of an inherent change of heart in the American people, but because it's the prized institution of the perceived enemy. A much more bitter ARW leads to stronger opinions on slavery being solidified and trickling down from intellectual back-and-forth to tangible ideology. By the end of the war the United States are in part defined by their anti-slavery stance. Due to the much more difficult war compared to OTL plus the fact that the slave population is overwhelmingly under British control, it's the Americans who had to make use of inciting slave revolts to bring down the British in the South. Whether inadvertent or not, America's ship is now tied to anti-slavery by the end of the war.
I'm not going to say that it's going to change American views on race overnight by any means, but I don't think it's that outlandish. The divides between the two groups were there from the very beginning of the United States, making the planters an enemy earlier when the United States is first forming a national identity should have a lasting impact. I skimmed over it earlier because it was more about achieving the OP's goal than writing a full outline, but this scenario absolutely involves threading the needle to win a much tougher ARW as a basis.